The present invention relates generally to data transmission. More particularly, the present invention relates to signal encoding to allow for the transmission of multiple digital signals via a single physical medium.
The amount of information that can be transmitted between two devices is generally limited by the bandwidth of the physical medium interconnecting the two devices. For example, the amount of digital data that may be transmitted between two integrated circuits (IC) is limited by the bandwidth of the physical wire connecting the ICs.
It is often desirable to send two or more simultaneous data streams between two devices along a single physical medium connecting the devices. For example, a first data stream may be a high rate primary signal and a second data stream may be a low rate secondary signal. The secondary signal may be used, for example, to transmit control information between the devices. However, as described above, the bandwidth of the physical connection between the devices limits the amount of data that may be transmitted.
Data encoding techniques exist which allow for the transmission of both a primary digital signal and a secondary digital signal over a single physical medium. For example, U.S. Pat. No. 4,716,563 entitled Demodulation of Auxiliary Low Frequency Channels in Digital Transmission Systems and U.S. Pat. No. 4,972,408 entitled Method and Apparatus for Combining and For Separating a Low Data Rate Digital Channel with or from the High Data Rate Digital Channel of a Transmission Link, both disclose techniques for transmitting a low data rate auxiliary digital signal along with a high data rate primary digital signal on a physical transmission medium. Both techniques modulate the low data rate signal so that it may be transmitted along the single transmission medium without interfering with the primary signal. One disadvantage of such techniques is that they require modulation of the low data rate signal at the transmitter and demodulation of the low data rate signal at the receiver, thus increasing the cost of the overall system.
In accordance with the invention, two or more digital signals are encoded using two or more respective line codes, such that the power spectral densities of the encoded signals are substantially orthogonal in the frequency domain. Since the power spectral densities of the encoded signals are substantially orthogonal, the encoded signals may be combined and transmitted via a single physical medium with little or no interference. Appropriate line codes are chosen taking into account the data rates of the two or more digital signals in order to achieve the desired orthogonality.
In accordance with one embodiment of the invention, a low data rate digital signal and a high data rate digital signal are transmitted via a single physical medium. The low data rate digital signal is encoded using a polar non-return-to zero line code and the high data rate digital signal is encoded using a Manchester line code. Due to the characteristics of these line codes, the encoded low data rate digital signal will have high power spectral density in a low frequency range and the encoded high data rate digital signal will have high power spectral density in a high frequency range. Further, the encoded low data rate digital signal will have low power spectral density in the high frequency range and the encoded high data rate digital signal will have low power spectral density in a low frequency range. As such, the power spectral densities of the encoded signals are substantially orthogonal and the encoded signals may be combined and transmitted via a single physical medium with little or no interference.
In accordance with another embodiment of the invention, a first high data rate digital signal, a second low data rate digital signal, and a third low data rate digital signal may be line encoded in accordance with the invention and transmitted via a single physical medium. In this embodiment, the first high data rate digital signal is encoded using Manchester line coding, the second low data rate digital signal is encoded using polar non-return-to-zero line coding, and the third low data rate digital signal is encoded using alternate mark inversion line coding. As a result of encoding, the power spectral densities of the encoded signals are substantially orthogonal and the encoded signals may be combined and transmitted via a single physical medium with little or no interference.
In accordance with one embodiment of the invention, a transmitter for encoding the digital signals includes line coders for line encoding received digital signals and a combiner for combining the encoded signals and transmitting the encoded signals via a single physical medium. A receiver for receiving the combined signal includes filters for extracting individual encoded signals and line decoders for receiving the individual encoded signals and decoding them to produce the original digital data signal. Since the line encoded signals have high power spectral densities in different frequency ranges, appropriate filters may be chosen to pass the appropriate encoded signal to the appropriate line decoder.